iov_iter: convert import_single_range() to ITER_UBUF

Since we're just importing a single vector, we don't have to turn it
into an ITER_IOVEC. Instead turn it into an ITER_UBUF, which is cheaper
to iterate.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

lib/iov_iter.c

@@ -1870,9 +1870,7 @@ int import_single_range(int rw, void __user *buf, size_t len,
 	if (unlikely(!access_ok(buf, len)))
 		return -EFAULT;
 
-	iov->iov_base = buf;
-	iov->iov_len = len;
-	iov_iter_init(i, rw, iov, 1, len);
+	iov_iter_ubuf(i, rw, buf, len);
 	return 0;
 }
 EXPORT_SYMBOL(import_single_range);